1. Technical Field
The invention relates to a quadrature modulator, a radio communication device comprising a quadrature modulator, and a method of calibrating the quadrature modulator or radio communication device.
2. Description of the Related Art
In a quadrature modulator, carrier leakage reduction and side band rejection have typically been carried out using one of two conventional methodologies. One of these methodologies depends on circuit matching, dynamic swapping, and the use of polyphase filters, all of which are carried out in the design phase. The second type of methodology deals with an imperfect chip but relies on user calibration methods while the chip is in use.
One example of the second methodology is disclosed in U.S. Pat. No. 6,169,463 to Mohindra et al. which discloses a quadrature modulator with set and forget carrier leakage compensation. Upon the power-up of the quadrature modulator, carrier leakage is measured in the in-phase and quadrature branches through the use of a synchronous detector. A state machine starts signal generators which inject compensation signals into the in-phase and quadrature branches, respectively, so that DC offsets in these branches are reduced thereby reducing the carrier leakage. Mohindra does not, however, disclose the rejection of undesired side bands. Mohindra proposes a simple detection scheme which can be difficult to implement since it does not distinguish between carrier leakage and side-tones or other errors. Also the large dynamic range of the signals that have to be suppressed require cumbersome and slow “gain-set” and “error-detect” sequences that have to be implemented in software.
It is desirable to not only suppress the carrier leakage but also to reduce undesired sidebands using minimal, and low cost, circuitry. It is also desirable to minimize the search for appropriate circuit parameters to reduce these undesirable circuit parameters.